Many people have a low opinion of online courses, and if the course doesn't have a teaching practice element, it will be regarded by many as second-rate, or worse. Some providers are trying to offer decent courses, but there are so many bad ones that they have a bad reputation. There are, however, blended courses, like the CELTA online, and some include video observation. If it does't have video observation as a very minimum, then the course is not worth much in the eyes of many employers. If a teacher hasn't been observed working in a classroom, why should an employer consider letting them loose on their students? Many won't.
To be honest, I would recommend the CELTA or an equivalent course (around 120 hours of training and 8 hours teaching)- that's what employers are looking for.
I looked to see if I could find any employers looking for people with online qualifications and couldn't find a single one, though I did find people who wouldn't consider applicants with them:
CELTA vs Online TESOL and TEFL Courses
60% of jobs I looked at on another occasion specified the CELTA or the Trinity College course.
Some qualifications are more equal than others - Tdol's English Language Blog
Student or Learner